This invention relates to curable propellant compositions and to a catalyst system for promoting their cure. In a more specific aspect, this invention concerns itself with a system for effecting the in situ formation of a catalyst during the isocynate curing of hydroxy terminated polybutadiene propellants.
Solid propellant compositions are generally composed of an oxidizer and a finely divided fuel component. In addition, plastic, resinous or elastomeric materials are often utilized as a combination fuel and binder for holding the propellant mixture together before the combustion reaction takes place. Polyurethane materials provide excellent binder materials and are extensively employed in various solid propellant compositions.
Urethane propellants are normally cured at temperatures of 110.degree.-135.degree. F. However, reduction of the curing temperature to 70.degree.-80.degree. F. would significantly reduce propellant bore strains and bond stresses in case bonded solid rocket motors. The lower cure temperature also results in better propellant mechanical behavior, because side reactions are also minimized at lower temperatures. Ambient temperature curing is doubly important for propellants containing energetic fuels for binder components, which may decompose during cure at higher temperatures. A considerable research effort, therefore, has evolved in an attempt to develop catalysts which promote the isocyanate-hydroxy reaction at room temperature under the conditions expected in a solid propellant environment. The catalyst must promote the isoycenate cure without seriously affecting propellant processing characteristics, mechanical behavior and storage stability.
A satisfactory balance between potlife and the time required for full cure is another major problem of ambient cure during which the accelerating effect of higher temperature on the urethane reaction cannot be utilized. This is particularly true for lithium initiated, hydroxy terminated polybutadiene (Li HTPB) prepolymer cured with a diisocyanate both NCO groups of which possess equal reactivity, for example hexamethylene diisocyanate. A conventional catalyst often employed in curing polyurethane is ferric acetylacetonate, an iron chelating agent, hereinafter designated Fe(AA.sup.1).sub.3.
At catalyst levels as low as 0.001% Fe(AA).sub.3, however, the propellant mix will be castable for only 30 minutes in case of hexamethylene diisocyanate and somewhat longer for toluene diisocyanate cured propellants but still require 7-10 days for full cure. In the absence of a catalyst, Li HTPB propellants are practically uncurable (weeks at 180.degree. F. are required). Reduction of the catalyst level below 0.001% entails the danger of losing the catalyst in degradative reactions. For these reasons some effort was spend searching for a catalyst of moderate activity which could be used in larger concentrations.
A highly satisfactory catalyst system was developed to solve the problems encountered in the ambient curing of polyurethane binders and propellants. This system provides for the in situ formation of the catalyst during the curing reaction.
With the present invention, it has been found that the in situ formation of an effective catalyst during the curing reaction can be accomplished by using a mixture of zinc oxide and a diketone or an organic acid, such as linoleic, .alpha.-bromotetradecanoic or p-tolueresulfonic. This system has been proven to be effective for lithium initiated hydroxy terminated polybutadiene propellants and even more effective for free radical initiated, hydroxy-terminated polybutadiene propellants, such as R-45M. In R-45M propellants, owing to the large excess of hydroxyl groups over the NCO groups of the Li HTPB propellants; side reactions are minimized.
The important consideration of the catalyst system of this invention is to bring about a temporary delay of catalytic activity, to insure adequate pot life, followed by reactivation of the catalyst to promote and ensure an adequate cure in a reasonable length of time. The present catalyst system does not have any deleterious effects on processing, aging and/or mechanical properties of the resulting propellant. On the contrary batch fluidity and thus, castability, are at a maximum because polymerization is delayed to a later stage, and the resulting mechanical properties are as good as any obtained by alternate methods. In addition, these methods are rather flexible, in that they allow catalyst activity to be modified at any stage of propellant processing.